C++构造函数中不能调用虚函数

//示例1
#include <iostream>
using namespace std;
 
class A
{
private:
    int m_a;
public:
    A(int a): m_a(a) {show();}
    virtual ~A() {};
    virtual void show() const {cout << "show a: " << m_a << endl;}
    virtual void disp() const {cout << "disp a: " << m_a << endl;}
};
 
class B: public A
{
private:
    int m_b;
public:
    B(int a, int b): m_b(b), A(a) {}
    void show() const {cout << "show b: " << m_b << endl;}
    void disp() const {cout << "disp b: " << m_b << endl;}
};
 
int main()
{
    A* pob3 = new B(100, 200);
    pob3->disp();
    delete pob3;
    return 0;
}

//示例1运行结果
show a: 100
disp b: 200
请按任意键继续. . .

//示例2
#include <tchar.h>
#include <iostream>
using namespace std;
class A
{
public:
	A()
	{
		cout << "A构造函数";
		Test();
	}
	~A()
	{
		cout << "A析构函数";
		cout << "A::Test()" << endl;
	}
	virtual void Test()
	{
		cout << "A::Test()" << endl;
	}
};

class B :public A
{
public:
	B()
	{
		cout << "B构造函数";
		Test();
	}
	~B()
	{
		cout << "B析构函数";
		Test();
	}
	virtual void Test()
	{
		cout << "B::Test()" << endl;
	}
};

int _tmain(int argc, _TCHAR* argv[])
{
	A* pA = new B();
	cout << "动态调用：";
	pA->Test();

	delete pA;
	return 0;
}

//示例2运行结果
A构造函数A::Test()
B构造函数B::Test()
动态调用：B::Test()
A析构函数A::Test()
请按任意键继续. . .

//http://www.cnblogs.com/vcpp123/p/5795421.html